1. Field of the Invention
The present invention relates to a method and apparatus for image forming of effectively detecting image data on a belt member. More particularly, the present invention relates to an image forming apparatus for detecting a patch pattern formed on a belt member and a method of detecting image data while keeping an appropriate focal length of a sensor for detecting the patch pattern.
2. Discussion of the Related Art
An image forming apparatus forms a visible image on an image bearing member and transfers the visible image onto a transfer member. The transfer member includes a sheet-like recording medium contacting the surface of the image bearing member for directly receiving a multiple color image, a belt-shaped member for receiving and carrying a multiple color image on a surface thereof as an intermediate transfer member, and so forth.
In a background color image forming apparatus, a plurality of image bearing members, which form and carry respective images corresponding to respective colors obtained after color separation and a belt member that serves as an intermediate transfer member or as a sheet carrying member, are disposed opposite to the plurality of image bearing members.
In a case where the belt member is mounted as an intermediate transfer member, the color image forming apparatus performs a primary transfer process for sequentially transferring and overlaying the respective separated color images formed on the plurality of image bearing members onto the intermediate transfer member and a secondary transfer process for transferring the overlaid color image onto a recording medium or recording sheet.
In a case where the belt member is mounted as a sheet carrying member, the color image forming apparatus causes the respective images formed on the plurality of image bearing members to be overlaid on the recording sheet while the recording sheet is carried by the sheet carrying member and conveyed between the sheet carrying member and the plurality of image bearing members disposed to face the recording sheet.
For accurately forming a color image with the above-described image forming operations, it is required that a color image forming apparatus obtain stable image forming qualities including color reproducibility without color shift, density nonuniformity, and so forth.
To achieve the above-described stable image qualities, a density detecting patch pattern image or a patch pattern may be used as corresponding image data.
The image forming apparatus forms a patch pattern, which includes a density detecting pattern, on an image bearing member or an intermediate transfer belt carrying a visible image transferred from the image bearing member. The patch pattern is optically scanned, and the color image forming apparatus feeds back the scanned result to control various parameters used for image forming conditions.
The feed back control in this case causes an image density detection sensor to measure an adhesion amount of toner on a patch pattern on the intermediate transfer belt. When the measurement result does not meet a predetermined condition, the feed back control is performed so that the various parameters can meet the predetermined condition. The various parameters include the write and read characteristics, the charging characteristic of the image bearing member, the charging characteristic for adhesive property of toner in the developer, the development bias characteristic that controls an amount of adhered toner, and so forth.
The patch pattern having a size larger than a detectable range of the image density detection sensor is formed on the intermediate transfer belt. The measurement of the patch pattern is performed with respect to the portion in which the output result from the density detection sensor is saturated, that is, the portion in which the patch pattern is formed in the entire detectable range of the image density detection sensor. Based on the detection result, the amount of adhered toner on the patch pattern is calculated. The calculated amount of adhered toner on the patch pattern is used to determine a predetermined density.
The patch pattern has a constant density and is formed outside the regular image forming area of a color toner image. Specifically, the patch pattern is formed between toner images of the regular image forming operation and has a predetermined distance from the leading edge of the next toner image so that the patch pattern does not overlap with the next toner image. When the density of the patch pattern is detected, a secondary transfer unit may be separated from the intermediate transfer belt.
Further, when detecting the density of a patch pattern, an optical sensor that is disposed opposite to a tensioned area of the intermediate transfer belt is used.
For a sensor used for detecting a patch pattern formed on a belt member, a reflective light sensor is commonly used because the reflective light sensor has an advantage in detecting the amount of reflected light according to the density of a patch pattern.
The reflective light sensor needs to maintain an accurate focal length with respect to a target material. When the focal length is not accurate, the sensor characteristic may change to deteriorate the detection accuracy, which may exert an adverse affect on the image forming control. Therefore, when the reflective light sensor detects the density of a target material, the focal length needs to be properly set or adjusted.
Further, vibration that may occur to a belt member while the belt member is moving may cause changes in image density and color registration. Vibration may occur while a belt member carrying a patch pattern to be detected is moving or the sensor output results obtained by a reflective light sensor may vary due to the surface characteristic of the belt member. Such vibration of the belt member or variation of the sensor output results can induce an unstable image density control and an unstable color shift control.
To eliminate the above-described problems, background image forming apparatuses have employed various techniques or methods as follows:
Method 1. A reflective light sensor detects patch patterns on a belt member moving on a suspension roller to which a smaller degree of belt vibration may be produced;
Method 2. A reflective light sensor detects patch patterns in an area on which the normal direction of a belt member crosses a suspension roller; and
Method 3. A reflective light sensor detects patch patterns on a rotatable supporting member that is disposed between suspension rollers.
When Method 1 is employed, the belt member serving as an intermediate transfer belt may produce less vibration of the sensor output results. Method 1 may, however, cause a sharp peak in predetermined intervals of the sensor output results when a foreign material gets caught between the intermediate transfer belt and the suspension roller.
Further, when the suspension roller is not sufficiently arranged in parallel or in a horizontal direction, the suspension roller may become eccentric when rotated, which can cause the sensor output results to vary in a rotation cycle of the suspension roller.
Aside from the variation of the sensor output results, when the diameter of the suspension roller is relatively small, a small degree of displacement of the sensor may change the distance of the detection surface of the intermediate transfer belt and the sensor to separate far apart, which can cause an adverse affect on a distance characteristic of the sensor.
When Method 2 is employed, no objects are caught under the intermediate transfer belt, and the surface of the intermediate transfer belt can be maintained in a relatively flat manner. Further, since the suspension roller is disposed in the vicinity of the target location, the amount of belt vibration in a vertical direction can be reduced.
However, if a slippage prevention guide, which is a rail-shaped guide for one cycle of the belt, is disposed at the back side or inner surface of the belt, the belt may be vibrated when the seam or boundary gap of the slippage prevention guide passes the suspension roller, and the sharp peak may be generated in every belt cycle.
Method 3 can provide a constant focal length of the intermediate transfer belt and the reflective light sensor.
However, similar to Method 1, when the rotatable supporting member is not sufficiently arranged in parallel or in a horizontally direction, the rotatable supporting member may become eccentric when rotated, which can cause the sensor output results to vary in a rotation cycle of the supporting member.